Added explanation

Author: Soupstraw

message-index/messages/GHC-58008/gadt-without-monobind/after/GADTWithoutMonoBind.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE MonoLocalBinds #-}
+
+module GADTWithoutMonoBind where
+
+data A a where
+  A :: A ()
+
+f :: A () -> ()
+f A = ()

message-index/messages/GHC-58008/gadt-without-monobind/before/GADTWithoutMonoBind.hs

@@ -0,0 +1,7 @@
+module GADTWithoutMonoBind where
+
+data A a where
+  A :: A ()
+
+f :: A () -> ()
+f A = ()

message-index/messages/GHC-58008/gadt-without-monobind/index.md

@@ -0,0 +1,24 @@
+---
+title: Pattern matching on GADTs without MonoLocalBinds
+---
+
+## Message
+
+```
+GADTWithoutMonoBind.hs:7:3: warning: [GHC-58008] [-Wgadt-mono-local-binds]
+    Pattern matching on GADTs without MonoLocalBinds is fragile.
+    Suggested fix:
+      Enable any of the following extensions: GADTs, TypeFamilies
+  |
+7 | f A = ()
+  |   ^
+```
+
+## Explanation
+
+When using GADTs, the type system is different in the sense that it's not 
+possible to derive the principal type of the function `f`. Both
+`f :: A a -> ()` and `f :: A () -> ()` can be considered the most general
+typing for the function. The `MonoLocalBinds` extension uses a slightly more
+conservative policy in order to be able to derive the type properly in such
+cases.